The modern greenhouse: film, glass and climate control
Hold the crop's temperature, humidity, light and carbon dioxide within range all year, at a heating and cooling cost worked out before the house is built.
Everything for this step
- Field guide
- Greenhouses → 15 sections
- Reference sheets
- None.
- Sciences it rests on
- Physics · Biology · Engineering · Agricultural science
- Track
- Protected growing, level 2 of 3 · next: Soilless growing: hydroponics and substrates
Why this matters
Under cover, and with its climate held steady, the same ground yields many times more. In 2024 the world's tomatoes averaged 3.7 kg per m²; the Netherlands, which grows them in heated glasshouses, averaged 47.9 kg per m² (FAOSTAT). The price is energy and investment: energy is on average 10–30 % of a greenhouse's production cost and can reach 40 %, and a heated house in Scandinavia uses about 1,900 MJ per m² a year against 500–1,600 in the Mediterranean (FAO, 2013). Most protected cropping therefore buys the gain cheaply: Almería in Spain grows vegetables under about 27,000 ha of low-cost film houses with no heating and little control beyond their vents.
The shape of it
- Size the climate before the structure. Heating is needed wherever the average minimum outside falls below 10 °C, and cooling beyond ventilation where the average maximum passes 27–28 °C (FAO).
- Choose the cover by light, heat loss and life: glass passes 88–94 % of the light and lasts over 25 years; double film 74–77 % for 3–5 years; twin-wall polycarbonate about 83 % for 20–25 years (University of Arkansas).
- Vent 15–30 % of the floor area, at the ridge and the sides; an insect net over the vents cuts the airflow by 40–70 %, so add area to match.
- Heat for the design night: heat loss is the cover's U-value × its area × the temperature difference. Each degree lower on the thermostat saves about 10 % of the fuel.
- Keep humidity below 95 % at night, or grey mould (Botrytis) follows.
- Count pests before you spray, and keep out of a sprayed house for the label's full re-entry time.
- Keep a standby generator and alarms: one cold night without power can kill a heated crop.
Doing it, step by step
The summary above says what has to happen. This says how, with the numbers that decide whether it works.
1 · Decide what it is for, and whether it pays
Name the crop, its day and night temperatures, and the months it will be under cover. A greenhouse pays for:
- Transplants and seedlings: high value per square metre, grown in the weeks before the field season.
- Season extension: the first and last crops of the year, when prices are highest.
- Warm-season fruit in a cool climate: tomato, cucumber, pepper.
- Winter salad and greens, which need little heat.
Then check the climate against FAO's thresholds: most greenhouse crops are warm-season plants that grow at 17–27 °C and stop below 10 °C or above 35 °C. Where the average minimum outside falls below 10 °C the house needs heat at night; where the average maximum passes 27–28 °C it needs cooling beyond its vents.
2 · Choose the type
Pick the cheapest house that holds the crop's range in your climate.
- High tunnel (hoop house): steel bows and one layer of film, roll-up sides, usually unheated. Adds weeks at both ends of the season. Build.
- Multi-span film house: tunnels joined at the gutters, with roof vents; can take fans, heat and screens.
- Glasshouse (Venlo type): glass roofs on steel, with vents in the roof; the house for heated, year-round production in cold climates.
- Passive solar greenhouse: a massive wall on the side away from the sun, film on the sunny side, an insulated back roof and a blanket over the film at night. Grows winter vegetables with little or no heating. Build.
The greenhouse guide compares them by cost, light, life and weak point.
3 · Site, orient and space it
Full sun, shelter from the wind, water, power and drainage. Build on a pad raised a little above the ground around it, sloping 1–2 % (University of Nebraska).
- Orientation: a single house with its long axis east–west lets in more winter light: about 13 % more than north–south with a 30° roof at 37° N (FAO). Run the crop rows north–south so light falls evenly across them.
- Wind: put roof vents across the prevailing wind, and the narrow end of a tunnel into it.
- Width: a house cooled only by its vents should be no wider than about 50 m, or a hot, still zone forms in the middle; leave space between houses for hot air to escape (FAO).
4 · Frame, cover and loads
The roof sets both the light and the snow load. Slope it at least 25–30° for winter light (FAO); a pitch of 6:12 (26.6°) or a pointed gothic arch also sheds snow (University of Vermont).
- Film: greenhouse grade, 80–200 µm, with UV stabiliser; add anti-drip and diffusing grades where you can. It lasts about 4 years (FAO: about 45 months).
- Glass lasts over 25 years and weighs about 10 kg per m² at 4 mm, against 184 g for 200 µm film (FAO), so its frame is far heavier.
- Anchor it: tunnel ground posts are driven at least 60 cm into the ground, deeper in sand (Noble Foundation); brace every corner.
- Snow: heat an occupied house so snow melts as it falls; on an unheated one, clear it early, evenly from both sides, and prop the ridge purlin in winter.
FAO advises replacing a greenhouse after 15–20 years. The guide has the cover table.
5 · Ventilate and cool
Vents first: 15–30 % of the floor area, at the ridge and the sides (FAO). Flap vents facing the wind move the most air: taking them as 100, flaps facing away move 67 and roll-up roof vents 28.
Fans: exhaust fans giving at least one air change a minute hold the house about 4.4 °C (8 °F) above the outside air in full sun (University of Georgia). Put them on the lee side, no more than 8–10 m apart, with an inlet opposite at least 1.25 × the fan area and air no faster than 0.5 m/s at the crop (FAO).
Shade: white shade cloth of 30–50 % or a washable whitewash on the cover, in the hottest months only (University of Vermont).
Fan and pad (drawn below) cools by evaporation where the air is dry: about 1 m² of wet pad per 20–30 m² of floor, 100–200 mm thick, air through it at 0.75–1.5 m/s, fans no more than 30–40 m from the pad, and 120–150 m³ of air per m² of floor per hour. Wet the pad before starting the fans; stop the fans before the water (FAO).
6 · Heat for the design night
Design temperature: the average daily minimum of the coldest month, less 8 °C (University of Georgia: less 15 °F).
Heat loss = U × A × (inside − outside), where A is the whole cover area and U its heat-loss coefficient: 6.0–8.8 W/m²K for single glass, 4.2–6.0 for double film, 4.2–5.0 for double acrylic (FAO, from ASAE). Air leaking in adds more: a new double-film house changes its air 0.5–1 times an hour, old glass in poor repair 2–4 times (University of Georgia). The guide works an example.
- Heaters: unit heaters each serve 180–500 m²; hot-water pipes at crop level put the heat where the plants are.
- Every combustion heater needs a flue to the outside and fresh air to burn. A ventless heater, or a cracked heat exchanger, fills a closed house with carbon monoxide and ethylene (Cornell e-GRO). Fit a carbon monoxide alarm.
- Save fuel: a thermal screen drawn across at night; each 1 °C lower on the thermostat saves about 10 % (FAO).
- Standby generator: at least 1 kW per 200 m² of floor to keep fans, controls and burners running (FAO); start it every week.
7 · Humidity, light and carbon dioxide
Humidity: 60–90 % does crops little harm; above 95 % for long periods, especially at night, grey mould (Botrytis) spreads. Bring it down to 70–80 % at dusk by venting with a little heat, and use anti-drip film so condensation runs off instead of dripping on the crop (FAO).
Light: a greenhouse loses 30–50 % of the daylight to its cover and frame. Aim for a daily light integral of 20–30 mol/m² a day for tomato and cucumber, 12–17 for lettuce and 5–10 for seedlings (Virginia Tech); add lamps where winter falls short.
Carbon dioxide: a closed, poorly vented house can run 20 % or more below the outside air by midday. Enrichment to 700–900 µmol/mol raises yields where the house can be kept closed; if the CO₂ comes from burning fuel, watch for nitrogen oxides and sulphur dioxide, which damage crops (FAO).
8 · Water, feed and pollinate
Water at the root with drip lines, and feed through them. Move air with circulation fans so leaves dry and humidity stays even.
Tomatoes under cover need help to set fruit: bumblebee hives work the flowers better than vibrating the trusses or spraying growth regulators, and a hive stays active for 6–8 weeks. Spray only products that spare them (FAO).
9 · Keep pests out, count them, and let their enemies work
Exclude, monitor, then act. Screen the vents with insect net, and remember it cuts ventilation. Hang yellow sticky cards just above the crop, near doors and vents, and blue ones for thrips; count and record the catch every week (FAO).
Release natural enemies early, before numbers climb: Encarsia formosa against greenhouse whitefly, Phytoseiulus persimilis against spider mite, Aphidius colemani against aphids, predatory mites such as Neoseiulus cucumeris against thrips (FAO). Use the farming guide for pesticide safety.
In a closed house, keep out for the label's full restricted-entry interval after a spray, as the US EPA's Worker Protection Standard requires.
What it looks like
Build it
Dimensioned drawings, bills of materials and construction procedures for the things this step asks for.